The 90 kDa heat shock proteins (Hsp90) are responsible for the maturation of approximately 200 client protein substrates, most of which are associated with signaling cascades that regulate cellular growth and proliferation. Therefore, Hsp90 inhibition provides a novel approach toward the treatment of cancer as numerous signaling cascades can be derailed through inhibition of the Hsp90-dependent protein folding process. There are four Hsp90 isoforms. However, the ability to selectively inhibit each of these has not been realized. Through collaborative studies, we propose to develop selective inhibitors of Hsp90 isoforms through rationally designed analogues that bind to the N-terminal ATP-binding site, that selectively disrupt Hsp90/co- chaperone interactions, and through modification of the natural product, geldanamycin. Such approaches are likely to afford compounds that exhibit greater selectivity, reduced toxicity, and identify isoform-dependent client protein substrates. Culmination of such data will provide a platform on which further isoform-selective inhibitors can be pursued for the development of new cancer chemotherapeutics.